\bibitem{Devreux90}     F.Devreux, J.P.Boilot, F.Chaput, B.Sapoval  \hskip 2mm
                        {\it NMR Determination of the Fractal Dimension in Silica Aerogels},
                        Phys.Rev.Lett. {\bf 65}, 614-617, 1990.